The present invention relates to a method for the preparation of a single crystal of silicon with decreased crystal defects or, more particularly, to a highly efficient method for the preparation of a single crystal rod of semiconductor silicon by the Czochralski process in which the number of crystal defects is remarkably decreased.
One of the very serious problems in the manufacture of semiconductor devices in a process including patterning on a single crystal wafer of semiconductor silicon is adsorption or adhesion of various particulate materials on the wafer because breakage of device pattern is sometimes caused thereby. This problem is particularly serious when the fineness of patterning is so high as to have a line width of 0.3 .mu.m or even smaller in the patterning for the preparation of a device utilizing the leading technology such as 64M DRAMs because a particle having a particle diameter of 0.1 .mu.m or even finer may cause a breakage of device patterning to greatly decrease the yield of acceptable products. Accordingly, prevention of particle deposition on the silicon wafer is an important problem to be solved.
In view of the above mentioned problem of particle deposition on the silicon wafer, the manufacturing process of silicon wafers is under a very strict control by using a particle counter with an object to uncover the sources of particle generation, to check the effectiveness of cleaning, to monitor the cleanness of the clean room, to inspect the final products of silicon wafers before shipping, and so on.
In the counting procedure of particles by the use of a conventional particle counter, the surface of a silicon wafer is irradiated spotwise with a laser beam having a diameter of 10 to 100 .mu.m under scanning and the very faint light scattered by the particles on the wafer surface is efficiently collected by means of a large number of optical fibers or an integrating sphere to convert the light into electric signals by means of a photoelectric device. In other words, a conventional particle counter works on a principle to count the number of the light points on the wafer surface caused by light scattering at the particles.
As is known, very small crystal defects are formed in a single crystal rod of silicon during the growth and the defects remain as such in the silicon wafers prepared from the single crystal rod through the wafer shaping process without annihilation during the procedure of cooling down of the single crystal as grown. When such a silicon wafer is subjected to a cleaning treatment in a cleaning solution which is a mixture of ammonia water and an aqueous hydrogen peroxide solution with an object to remove particles deposited thereon as is conventionally undertaken, a large number of pits are formed on the surface of the silicon wafer because the rate of etching is high in the crystal defects indicating generation of crystal defects. When a silicon wafer after such a cleaning treatment is subjected to counting of the particles adsorbed thereon by using a particle counter mentioned above, the true number of the particles can never be obtained because scattering of light, which is detected and counted as a particle by the particle counter, occurs also at the pits formed by the etching treatment.
It is a known trend that the number of crystal defects is much larger on a silicon wafer prepared by working from a single crystal silicon rod grown by the Czochralski (CZ) method than on a single crystal silicon wafer prepared by working from a single crystal rod obtained by the floating zone-melting (FZ) method or on a so-called epitaxial silicon wafer which is a single crystal wafer of silicon by the CZ method and provided with an epitaxially grown thin film of silicon on the surface.
It is also known in the prior art that the number of crystal defects introduced into a single crystal silicon rod during the single crystal growth can be greatly decreased if growing of the silicon single crystal by the CZ method is undertaken at a greatly decreased rate of, for example, 0.4 mm/minute or lower as is disclosed in Japanese Patent Kokai 2-267195. While the rate of single crystal growing of silicon is usually at least 1 mm/minute in the conventional process, this approach of greatly decreasing the crystal growing rate to 0.4 mm/minute or lower is not practicable because the desired decrease in the number of crystal defects can be accomplished only at a great sacrifice of the productivity to less than a half consequently with a great increase in the production costs even though the number of crystal defects could be decreased so far.